Fixing The Weakest Link

One of the soybean's most unique traits — that it fixes its own nitrogen — is also its weakest link during a drought.

But this fair-weather trait may have a champion in Larry Purcell, University of Arkansas crop physiologist. He is one of the researchers studying nitrogen fixation and its effect on soybean yield during drought stress.

Purcell hopes his and others' work will develop high-yielding varieties with drought tolerance. “Nitrogen fixation is very sensitive to drought stress. And it's very, very important for soybean yields and for the product people are trying to get from soybeans — high-quality protein,” Purcell says.

But during drought, soybeans can't fix nitrogen. Actually, the crop breaks down the protein in its leaves to supply the seed. This leads to early plant senescence or death — and yield falls off.

So Purcell; USDA crop physiologist Tom Sinclair, University of Florida; and soybean breeder Clay Sneller, then with the University of Arkansas, decided to look for a solution.

The researchers found an old soybean cultivar named Jackson, which has “substantially greater drought tolerance for nitrogen fixation than did several other cultivars,” Purcell says.

In the early 1990s, when Purcell was a University of Florida graduate student, he and Sinclair looked at the fundamental differences between the Jackson cultivar and drought-sensitive cultivars. When Purcell began his research at Arkansas, he and Sneller began making crosses of Jackson and drought-sensitive cultivars to look at the inheritance of the N-fixing trait.

“We found that it is fairly heritable; it can be passed from one generation to another,” Purcell notes. “That means we should be able to combine good yields in improved cultivars with germplasm that has drought-tolerant nitrogen fixation.”

One cross between Jackson and a Kansas cultivar, tested under 10 different drought-stressed environments, performed better than standard cultivar checks.

Now they're using molecular marker technology to see if they can determine which piece of DNA is actually associated with the drought-tolerance trait.

They're also hoping that molecular markers will help determine if specific traits contribute to drought tolerance. One trait Purcell and colleagues are curious about is nodule size. The cultivars offering drought-tolerant nitrogen fixation tend to have large nodules, Purcell says.

At the same time, the researchers are working to increase yields on the drought-tolerance lines they've been working with.

“We're carrying baggage from the old cultivar, Jackson,” Purcell admits. “But several of these lines look very good. The yields under irrigated conditions have not been as good as the checks, but in the drought conditions they perform as well or better than the checks.”

If the researchers are able to select for a nitrogen-fixing gene or genes, they're probably not more than three or four years away from releasing germplasm to breeders.

“We're still at the point where we're wanting proof to convince ourselves, as well as everyone else, that this is something that breeders can use in their own breeding programs.”

It's like finding a needle in a haystack “and showing that you can actually sow with the needle,” he adds.